THE LOWER SILURIAN ROCKS. 559 



area of Trenton occupies the high ground at the head of Sugar river, 

 in Cross Plains and Middleton, whilst the numerous narrow ridges 

 "between the branch streams of Sugar river in Primrose and Spring- 

 dale are everywhere crowned by this formation, which in the highest 

 ridges is present in its full thickness. The high Trenton area of 

 Middleton and Cross Plains constitutes the divide between the heads 

 of Sugar river and Black Earth creek. On the south side of the val- 

 ley of the latter stream it breaks down quite suddenly. Further 

 north, in northwestern Middleton, southwestern Springfield and south- 

 ern Berry, a few very small Trenton areas are met with. 



No very distinctive topographical characters mark the region occu- 

 pied by the Trenton limestone. Most commonly the areas underlaid 

 by it are prairie areas, and in some cases the coincidence of Trenton 

 and prairie areas is striking. In all cases the soil derived from it is 

 very fertile. In eastern Dane and Columbia, it occupies areas of 

 gently rolling to level country, whilst on the west side of Dane it 

 forms the rounded summits of steep and narrow ridges. Hardly ever 

 forming natural outcrops of any size, it contributes no especially pic- 

 turesque features to the scenery. 



The lithological characters of the Trenton limestone contrast 

 strongly with those of the Lower Magnesian, it being throughout very 

 evenly bedded, commonly close-textured, rarely cherty, and having 

 aluminous (clayey) rather than silicious (sandy) impurities. More- 

 over, though largely dolomitic, it includes a considerable thickness of 

 non-magnesian limestone, standing, in this respect, alone amongst the 

 Silurian limestones of the northwest. In the lead region, according 

 to Hall and Whitney, only the lower 18 to 20 feet of the Trenton are 

 dolomitic, constituting the "Buff" limestone of their and other re- 

 ports, whilst above., all of the remaining 50 to 80 feet of the forma- 

 tion are true limestone, into which a small and gradually increasing 

 amount of magnesia enters as the upper layers pass into the overly- 

 ing Galena. The lower of these divisions, the Buff limestone, with a 

 thickness of 25 feet, is well marked throughout Columbia and Dane 

 counties, as is also the lower portion of the Blue limestone immedi- 

 ately above. The higher portions of the formation, which have for 

 the most part been removed by denudation, and are hence but rarely 

 seen, do not seem to bear out Hall's and Whitney's descriptions, since 

 they certainly include some dolomitic layers, in appearance quite like 

 the Buff beds. The exposures of these higher beds are, however, so 

 infrequent, that I would advance this statement with some doubt, but 

 for the fact that in the Eastern Wisconsin district, where all parts of 

 the formation are well developed, Professor Chamberlin has made out 



